1. Field of the Invention
This invention relates to liquid purification methods and means employing freeze crystallization using multiple batch crystallization chambers wherein a liquid is partially frozen, the unfrozen liquid fraction is extracted, and the remaining crystalline phase is melted. Generally, the invention relates to systems using indirect cooling, that is, the coolant is separated from the liquid to be purified by the walls of the crystallization chamber, and direct heating, that is the frozen fraction is melted by circulation of warm previously purified liquid through the chamber containing the frozen fraction. More particularly, the invention relates to those systems where the purified liquid is warmed by the heat rejected from a heat pump which is extracted thermal energy from an alternate crystallization chamber, and to the use of multiple stages of heat pumping to reduce energy consumption.
2. Description of the Prior Art
A batch crystallization apparatus for liquid purification operates in a cycle consisting of the following steps repeated in sequence:
(a) fill a chamber with the liquid to be purified;
(b) remove heat from the chamber to form frozen liquid of the desired thickness;
(c) drain the unfrozen liquid containing the concentrated impurities from the chamber;
(d) melt the purified frozen liquid and drain it into a storage tank or vessel for use.
Because the solubility of impurities is much higher in the liquid phase than in the solid phase, the impurities become relatively concentrated in the unfrozen liquid and the crystalline solid (for example, ice) is relatively purified. The frozen liquid can be melted using electric heating, storage of the heat removed during the freezing step, or using the heat removed from a second chamber whose freezing step coincides with the melting step of tile first chamber.
For example, U.S. Pat. No. 4,799,945, issued Jan. 24, 1989 to Chang suggests using a vapor compression system whereby refrigerant evaporates in one chamber to form ice, is compressed and then condensed in a second chamber to effect the melting of ice previously formed. Dual crystallization chambers are operated alternately in freezing and melting fashions, with the refrigerant or a heat transfer fluid circulated on the outside of the chambers.
In the Chang system, the direction of heat addition and heat removal are coincident, which leads to certain practical difficulties. First, the ice melts away from the heat transfer surface, resulting in poor utilization of the available heat transfer area since only the lower portion of the chamber, where the melted liquid collects, has intimate contact between the ice and the surfaces being heated by condensing refrigerant. Second, the Chang method requires that the walls of the freezing chamber be alternately cooled and then heated to effect the melting, resulting in reduced production capacity and reduced efficiency.
Chang et al., U.S. Pat. No. 4,954,151, issued Sept. 4, 1990, describe a means of maintaining ice in contact with the walls of the chamber during the melting phase. In one embodiment of a batch purification chamber, ice is foraged on a horizontal cold plate which is in contact with the upper surface of the water to be purified. In this embodiment, after the unfrozen liquid fraction is withdrawn from the chamber, the purified ice sheet is held against the cold plate by springs, and then heat is applied through the cold plate.
Conlon et al., U.S. Pat. No. 5,113,664, issued May 19, 1992, describe a simplified system that eliminates the need for springs to hold the ice against the cold plate. In this system, the ice is allowed to fall from the cold plate onto a heated melter bar, where it splits into two pieces which are then directed by gravity against the heated bare by means of inclined mils against. However, either this system nor that of Chang et al. address vertically oriented freezing chambers.
Chang has also described means for alternating the flow of hot and cold refrigerant or hot and cold heat transfer fluid from one chamber to another to effect the melting. In one of Chang's embodiments, two vapor compressors, a heat pump, and a complex arrangement of refrigerant valves was used to transfer heat from one chamber to another and to reject heat to the environment. In another embodiment, Chang proposed a single vapor compressor with heat rejected first to the melting chamber and then to via a secondary heat exchanger to the environment, requiring that all of the heat removed from the freezing chamber is pumped to a high temperature sufficient for rejection to the environment. In a third embodiment, Chang proposed a single heat pump and intermediate heat transfer fluids for heating and cooling. In each of the embodiments, the same coil was used for both heating and cooling, resulting in inefficiency due to the need to add and remove heat from the coils themselves.